Ok, I lied a little bit. Sometimes caterpillars come out in the winter.
If you live in the US or Canada and see a fat brown caterpillar sauntering across the snow, it is probably Noctua pronuba, the Large Yellow-Winged Dart. It is an introduced species from Europe which has spread across much of North America. The caterpillars feed on all sorts of grasses and forbs, so they can be found just about anywhere (and if you find one, you can feed it just about anything). When we rear these caterpillars in the lab, we simply step outside the building and grab a handful of grass.
Why these caterpillars frolic in the snow remains a mystery. Many species in the same subfamily (Noctuinae) overwinter as larvae, and emerge to feed on warm days in early spring. However this species takes it a step farther, crawling and foraging on bitter cold days when no sensible caterpillar would be out and about. While most caterpillars wait until April or May to emerge, this species can be seen as early as January! Sometimes they even emerge en masse, hundreds of caterpillars wriggling on the snow.
Photo courtesy of Stan Malcolm
This strategy does not appear to hinder the species, as Noctua pronuba is one of the most abundant moths in many parts of its range. The caterpillars appear to be stimulated into action by one or two “warm” days, which may be just above freezing. Perhaps they have become overly sensitive to temperature changes, causing them to emerge in the snow instead of waiting until spring?
While it might not seem like there would be food available to a caterpillar in January in the Northeast, these caterpillars are not picky. Many of them attack roots of plants. As an agricultural pest, Noctua pronuba may feed on exposed annual plants if they are not covered by snow. This might provide enough of an advantage to the caterpillars that it balances out potential mortality due to the cold and predation (a brown caterpillar is pretty obvious on white snow). But studies would have to be done in order to say for certain.
Have you seen any caterpillars on the snow? Have you seen any that are notNoctua pronuba?
And does anyone know if they also appear on the snow in Europe, or is it a behavior expressed only in North America?
Further reading: _______________________________________________
-Wagner, D. 2011. Owlet Caterpillars of Eastern North America, Princeton University Press, pg. 538.
-Diagnostic Services at Michigan State University